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Anatomy of Fish

Body Shape

In common with most fishes, Koi are broader at the front than at the tail end. Such a shape streamlines the Koi and minimizes turbulence as it moved through the water. If a cone is placed with the pointed end turned into a current of water, the water swirls at the rear flattened surface, creating turbulence, or drag. If the cone is reversed, so that the flat surface meets the current, the flow of water around it is less turbulent and actually assists its passage through the water. The movement of Koi through the water is similar to that outlined for the second cone.

The External Anatomy

The external anatomy of Koi includes the scales, skin, fins, operculum (gill cover), vent, eyes, nostrils, barbels and lateral line.

Skin & Scales

A koi's skin is covered by a layer of mucus, which not only offers protection against disease but also helps streamline the fish to aid locomotion. This layer of mucus is continually being refreshed as the old mucus sloughs off into the water. Below the mucus lies the top layer of skin know as the epidermis. This is extremely thin and in fact lies above the scales. The epidermis is responsible for mucus production and assists wound repair by multiplying to cover areas of damage.

Under the epidermis is the dermis and this is where the scales are formed. The scales form a lightweight, pliable suit of armor over the surface of the fish. Scales make up a calcified flexible plate; they contain high levels of calcium and offer an extra line of defense. In times of nutrient depletion this calcium can also be used as an extra nutrient source. The front end of the scale is inserted into a deep pocket of skin, the back is quite free and overlaps the front of the scale behind, just as roof tiles overlap.

There are two distinct types of scale pattern found on Koi: completely scaled, and the so-called doitsu, in which the scales are enlarged and arranged along the midline of each side of the body and, usually at the dorsal and anal fin bases. The scales of doitsu are entirely embedded in the skin.

On some koi, the dermis grows from beneath the scale and is seen as ‘fukurin.’ Varieties such as ogon and asagi typically display fukurin especially on the shoulder area. It appears around the external edges of the scales.

The dermis also contains chromatophores, which are responsible for coloration, as well as nerves, blood vessels and sense organs.

The Immune System

The immune system comprises many elements, the first line of defense being the scales and the mucus which covers them. This has anti-bacterial and anti-fungal properties and is continually renewing itself. Next to the scales lie the dermis and epidermis which constitute the second layer of the fish's external barrier. Many infections take hold by entering through lesions in the skin. The digestive tract also serves as part of the immune system creating an environment that is unfriendly to pathogens.

If these defenses are breached and an infection enters the bloodstream, the next line of defense are the white blood cells which ingest the foreign bodies and carry them to the kidney and spleen. The spleen is responsible for the production of cells that are vital to the fish's immune system, and it also acts as a store for immature red blood cells. In the kidney and spleen antibodies are formed that fight the infection. If the fish has experienced the alien pathogen before, the fish's immune system will react quicker than if it has to combat a new intruder.

The speed of reaction also depends upon the water temperature; lower temperatures lengthen the response time of the immune system, while levels of pathogen activity may still be high. This is why pond heating is so beneficial to their healthcare.


Koi have five sets of fins: the dorsal, anal and caudal (or tail) fins, which are single, and the pectoral and pelvic fins, which are paired. Fish swim using the body muscle, and the fins play an important role in maintaining stability in the water. When erected, the dorsal and anal fins prevent rolling and yawing, and the pectoral and caudal fins control pitching. The pectoral and pelvic fins also control fine movement by counteracting the propulsive forward motion caused by the exhalation of water from the gill covers, thus enabling the Koi to remain motionless.

Operculum (gill cover)

This is a large bony plate that protects the delicate gills. It is free at the rear and lower edges and acts as a one-way valve, allowing water to leave the gill chamber but preventing any backflow of respired, oxygen-deficient water to re-enter the gills. It articulates with the skull and the large hyomandibular bone. (In fishes, this bone forms part of the suspension for the lower jaw.)


Just in front of the anal fin is a large pore, usually termed the vent. The gut and the ducts from either the testes or ovaries terminate at this point. This is not a cloaca, which is a term used to describe a common opening for the digestive urinary and genital systems. Just in front of the vent there is a smaller opening which receives the urinary ducts form the kidneys.


Koi, like many other fish species, have excellent eyesight. Many experts think that they actually see things in a very similar way to humans, and are able to make out shapes and colors.

The position of the eyes on the head gives them almost total 360⁰ vision, enabling them to see above and behind, as well as in front and below themselves. The ability to see above and behind is particularly important because it enables Koi to watch for the approach of enemies while they are feeding.


On either side of the snout are a pair of nostrils, four in all. In cross section, the nostrils are seen to be joined by a U-shaped tube. The water flows in through the front nostril and out through the rear one. The floor of the tube is folded into a series of ridges, which are then arranged in the form of a rosette. The ridges are coated in a skin that is peppered with olfactory, or smelling, cells. These cells are able to detect extremely small quantities of dissolved substances in the water, even from quite distant objects. In fact, the sense of smell is more important to the Koi than eyesight for finding food.


Koi have one pair of barbels on each side of the mouth, a small barbel on the side of the upper lip and a larger one more or less at the corner of the mouth. These fleshy structures are moved by muscles that once operated parts of the jaw and palate and are covered with taste buds that literally allow Koi to taste anything the barbels contact. Like their ancestors, the wild carp, Koi are bottom feeders, using their barbels to locate grubs, insects, crustaceans, worms and water plants in the substrate. (Koi keepers generally feed Koi with floating pellets and sticks, but this is simply to enjoy seeing their spectacular livery as they surface to feed). Thus, Koi use their sense of smell to locate an area where food is to be found, and then detect the individual particles of food using the very sensitive taste buds on their barbels.

Lateral Line Organ

Along the middle of each side of the body there is a row of scales, each pierced by a small pore that connects by way of a small tube to a canal that lies in the skin beneath the scales. This system of canals and tubes forms part of the lateral line organ. On the head, the lateral line organ runs beneath the eye to the snout, its passage being marked by conspicuous pits. The lateral line is a sensory organ used to detect vibrations in the water.

In some doitsu Koi, the lateral line may be seen as a conspicuous line that runs along the middle of the fish, from just behind the operculum to the tail.

Hair cells transmit any vibration via nerve fibers to the spinal cord which sends a signal to the brain. Although the sensory cells of the lateral line are similar to those found in our internal ear, it is difficult to interpret precisely the sensation these cells impart to the fish. It seems that they are sensitive to water movement, waves and disturbances in the water, enabling Koi to avoid other fish, objects and the pond walls.

The Internal Anatomy

The internal anatomy is more complex than the external anatomy, not only because of the number of different organs involved, but also the way these various organs interact.


One of the most important features of the internal anatomy, but one which receives the least consideration, is the skeleton. Koi are members of a group of fish known as teleosts, literally meaning 'bony skeleton'. The skeleton has several major functions: it forms a protective and supporting framework for the internal organs and tissues, and enables the fish to move by providing a series of flexible joints and attachment points for muscles.

Parts of the first four vertebrae of the backbone are modified to form the Weberian ossicles, a system of tiny bones that link the swimbladder to the inner ear. Sounds traveling through the water cause the swimbladder to vibrate and he ossicles amplify these vibrations and transmit them to the sensitive hair cells within the liquid-filled semicircular canals of the inner ear. With such and apparatus, Koi can undoubtedly 'hear'.


Koi, like all other members of the cyprinid family, lack teeth in the jaws. Their few teeth are large grinding devices on plates of bone in the throat.


The bulk of the Koi body is formed from four large blocks of muscle, two on either side of the body. These blocks are subdivided into V-shaped muscle segments. The muscle segments of the throat and gill arches are very well developed to control respiratory movements. The jaw muscles cover most of the cheeks and exert their force in closing the jaws. The muscles of the paired fins are simple in construction, although those controlling the pectoral fins are very extensive.


The heart, situated just behind and below the gill arches, is a large muscular pump consisting of four consecutive chambers. The first chamber is simply a thin walled sac with very little muscle. The second chamber, or atrium, is also thin walled, but expandable. The third chamber, or ventricle, has thick muscular walls and is largely responsible for the pumping action of the heart. The fourth chamber is also thick walled and has special non-return valves in it, preventing blood flowing back into the preceding chambers.

Circulatory System

This is the transportation system by which nutrients and oxygen reach the cells of the body and waste products are removed from them. It is an embracing term that includes the blood, plasma and vessels through which these body fluids flow. The heart provides the pumping force to move the blood through these vessels, first to the gills and then to the brain and the remaining parts of the body, before returning once again to the heart. The vessels that carry blood to and from the gills, to the brain and then body are arteries; those that return the blood to the heat are veins.

Generally, the most important function ascribed to the circulation is transporting oxygen from the gills to the tissues and removing carbon dioxide from the body via the gills. However, it also supplies a constant stream of food materials from the gut or liver to the tissues. As the tissues process the foodstuffs, they produce nitrogen-containing waste products, which are poisonous to the tissues. Some of these nitrogen products, together with any excess water, are transported in the blood to the kidneys, where they are extracted and processed into urine. As in other bony fish, most of the nitrogenous wastes leave the body in the form of ammonia, which diffuses into the water from the gills.


The blood consists of three main elements: the plasma, red blood cells and white blood cells. The plasma is a watery fluid of complex structure. The red blood cells contain haemoglobin, a red pigment that binds with oxygen. Thus, the red blood cells flowing from the gills carry oxygen to the tissues, where they exchange their oxygen for carbon dioxide, the waste gas produced during the process of metabolism.

The white cells in the blood have an equally important role to play, but mainly in terms of protecting the fish from infection. There are several different types of white blood cells, their precise function varying according to their type. All accumulate in injured, inflamed or infected areas of the body, where they repel bacterial or viral invasions, remove dead or damaged tissue, and assist in repairing damaged tissues.

Blood cells have a short lifespan, new ones being produced primarily in the spleen and kidneys.


The gills are the equivalent of our lungs; they provide a large surface area of tissue where carbon dioxide is exchanged for oxygen. In addition to carrying out this process of gaseous exchange, they are an important site for osmoregulation (controlling the salt/water balance and excretion).

To the naked eye, the gills appear as rows of very fine, finger-like projections arranged on a series of curved arches. There is a double row of delicate filaments in a V-shaped arrangement along the posterior surface of each arch. The surface of the filaments is highly folded, enormously increasing their surface area, and the skin is extremely thin, allowing intimate contact between the blood and water. As water flows over the gill filaments, oxygen is extracted and carbon dioxide released - a process of external respiration.

The osmoregulatory role of the gills is made possible not only by their large surface area but also by an outer layer of special cells that absorb salts from the water. (Salts is used here in the chemical sense, rather than simply referring to sodium chloride, or common salt.) Since the body tissues and fluids of a Koi contain a higher concentration of salts than the surrounding fresh water, there is a tendency for water to be absorbed and salts lost through the gills. (The processes at work here are osmosis and diffusion. Osmosis is the process by which water passes through a semi-permeable membrane - in this case, the gill membrane - from a weaker to a stronger solution. Salts move by diffusion in the opposite direction. Water and salt movement stops when the solutions are equally concentrated on either side of the membrane.) The excess water absorbed through the gills is excreted in copious amounts of dilute urine produced by the kidneys. The special cells in the gill membrane selectively absorb salts back from the water, thus helping to maintain the correct salt balance within the blood and body tissues of the Koi.


The brain is a very soft, pink organ encased by the bony skull. During the development of the embryo in Koi, the brain and spinal cord are formed as a tube. The front portion subsequently enlarges to form the brain, the hollow within the tube persisting as fluid-filled cavities, or vesicles. The brain consists of three regions: the fore-, mid-and hindbrain.

At the front of the forebrain are the olfactory lobes, one lobe for each pair of nostrils on each side of the head. They receive the nervous impulses from the nostrils and interpret them as smells. The forebrain also incorporates the light sensitive pineal organ and hormone secreting pituitary gland. During the development of the embryo, part of the forebrain is responsible for forming part of the eyes, their associated nerves and the pineal organ. (Interpreting optic nerve signals as sight is a function of the midbrain.) The pineal organ, located on the upper surface of the forebrain, is light sensitive and is thought to represent the remnant of a second pair of eyes on top of the head in some remote ancestor of vertebrate animals. In Koi, the pineal organ senses changes in day length and stimulates the release of reproduction as well as activating other glands, such as the thyroid.

The main function of the midbrain is to interpret messages relayed to it by the nerves, particularly those concerned with movement and the attitude of the body in the water. The midbrain also acts as the seat of memory, learning and intelligence, although it is very difficult to assess or quantify degrees of learning and intelligence in Koi. On top of the midbrain are two oval optic lobes that interpret nervous signals from the eyes and thus provide the sense of sight.

The function of the hindbrain is unclear, although it appears to act as a coordination center for movement, equilibrium and posture.

Anatomy Pictures

Nomenclature of some external features (fins)

Schematic view of gills and internal organs


Operculum (structure which covers the gills)

Schematic view of histologic features of gill arches in relation to water flow (after Reinert, 1992)

There is a set of 4 gill arches on each side of teleosts. 1° & 2° lamellae on each gill arch The thin epithelial layer lining the filamentous gill structures is very thin and allows gas exchange to occur here.





Adrenal Gland - The adrenal cortical tissue is represented by the interrenal cells. The adrenal medullary cells may vary is location.

Thyroid Gland - Thyroid follicles are very similar to mammalian thyroid tissue. Thyroid follicles are widely distributed throughout the viscera.

Pancreas - Islets of Langerhans may be grossly visible.

Here is a .pdf file you can download on Anatomy it has a lot of helpful information.

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The gut performs three major functions: physical breakdown and mixing of food (by the teeth); chemical breakdown (by digestive enzymes); and absorption of the food materials. In Koi, there is no stomach; food is digested in the very long intestine. Once the nutritive content of the food has been absorbed into the bloodstream, the remaining solid, undigestible material is voided.


The spleen, a compact purple-red organ, lies very close to the gut and liver. It is important for the formation and storage of red and white blood cells.


The liver in Koi is very large and consists of several lobes. Its main function is to store glycogen - a stored form of glucose - and, to a lesser degree, other food products. As glucose and other simple sugars are used up by the working tissues and organs of the body, the liver releases the stored glycogen, which is broken down into glucose and fuels the working cells.

The liver also breaks down old and damaged blood cells. The product is known as bile and collects in the gall bladder. From here it drains through the bile duct into the intestine, where it is mixed with the solid waste.


The pancreas is a soft tissue, similar in appearance to the liver, that produces a number of digestive enzymes, which it releases into the intestine to chemically break down food.


The kidneys are paired organs that lie in the dorsal (upper) region of the body cavity on either side of the vertebral column, or backbone. Each kidney is basically a construction of tubules, closely surrounded by a network of tiny blood vessels, called capillaries. The tubules are connected to a system of ducts that merge into a single one, the ureter, which runs from the kidneys to a pore just in front of the vent, where the urine is voided.

The intimate contact between the capillaries and the kidney tubules allows waste products to pass from the blood into the tubules in a continuous filtering process. The waste products involved are simple nitrogen compounds formed in the tissues during the metabolism of proteins. These compounds would prove poisonous to the cells if they were not carried away in the bloodstream and removed by the kidneys (and via the gills). The kidneys also regulate the water content of the body - urine is largely water - and control the type and quantity of salts retained by the Koi.

Reproductive organs (Gonads)

In males these are the testes; in females, the ovaries. These are located internally and are found on either side of the body.

The testes tend to be rather compact and more regularly shaped than the ovaries and, although subject to seasonal change, they show no marked difference between the resting and breeding condition. The testes produce sperm (milt) cells, many millions of which may be released during the breeding period.

The ovaries are relatively large and irregularly shaped organs in mature female Koi. Eggs may be found in the ovaries throughout the year but the number of mature eggs present is very low when the fish is not in breeding condition. When the female is ready to breed, the ripe eggs burst into a central cavity in the ovary, then pass into the oviduct and are shed at the vent.


As its name suggest, this organ helps the Koi to swim by providing buoyancy in the water. It is an elongated oval sac that lies along the top of the body cavity, just beneath the vertebral column and kidneys. It is almost subdivided into two parts by a narrow constriction. At this point there is a duct, which connects the swimbladder to the gut. Indeed, in embryo Koi the swimbladder develops as a minute outgrowth from the gut. This connecting duct enables Koi to top up the swimbladder by gulping and swallowing air at the water surface. The air bubble passes into the gut and is pumped in to the swimbladder by a duct. The mixture of gases within the swimbladder is more or less the same as air.

Spinal cord

The spinal cord is the posterior continuation of the brain, its fluid filled central canal being the vestigial remnant of the embryonic tube from which the brain develops. The spinal cord extends almost the entire length of the body and is connected to many of the major nerves. As in other vertebrate animals, its main role is to act as a through route for nervous impulses traveling between the body and the brain.

Nervous system

Apart from the brain and spinal cord, which can be considered as the central nervous system, a complex network of nerves extends throughout the body of the Koi. This system relays sensory information, such as touch, taste, and smell, to the brain and activates the tissues, glands and muscles. There are two quite clearly defined types of nerve systems: the autonomic system and the motor system. The autonomic nervous system controls the unconscious activity of organs such as the gut, gills and certain glands. In effect, the fish is unaware of their background functioning. The motor nerves, on the other hand, carry signals to initiate conscious activities, such as the contraction of muscles to achieve movement.

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